1. Field of the Invention
This invention relates to the art of pulse compression circuits and more particularly concerns a pulse compression generator employing digital circuits with finite steps to generate continuous wave shapes, usable in radar and similar systems.
2. State of the Art
Heretofore pulse compression generators usable in radar systems have always used passive analog circuits having lumped parameter networks, very accurately tuned. These circuits include crystal filters, vacuum tubes, solid state amplifiers, and tuned networks of precision resistors, capacitors, and inductors. Two basic objections to the prior pulse generators, particularly those usable for obtaining a desired signal or simulating radar pulses, are their high cost and great complexity. A further objection is their inordinately large size and weight. A typical high quality precision compressed pulse generator of conventional type has hundreds of components, and costs about $70,000. It is over four feet in length and weighs many pounds.
3. Brief Summary of Invention
The present invention takes another approach. Here a lightweight, small, relatively inexpensive and simple unit uses a digital circuit feeding a digital-to-analog circuit which drives an R.F. balanced modulator circuit to produce desired FM sweeps or pulse compression pulses. FM pulses or "chirps" are generated continuously by first obtaining from a radar manufacturer or end user a phase history of frequencies as a function of time for transmitted radar pulses or a desired signal. That is, the phase is varied according to a prescribed system. It consists of amplitude modulating two carriers exactly 90.degree. apart from each other, thereby generating one vector at 0.degree. and 180.degree. and a second vector at 90.degree. and 270.degree.. The two vectors are in phase quadrature. The phase addition of the two vectors gives the effect of rotating phase vector.
In the present embodiment of the invention, there is a single frequency reference source and a signal input applied to a 90.degree. phase shifter which yields the two phases components in quadrature. Each of the two phase components are applied to a balanced amplitude modulator and then combined in an output adder to produce the desired output wave shape. The required phase information for the modulators is obtained by digital means. A time scale is generated on a digital counter, the output of which is a time variable address. This address is applied to a Programmable Read Only Memory (PROM) or a Read Only Memory (ROM) containing the desired phase history of the signal. The output of the PROM or ROM, as the case may be, in turn addresses two PROMS, a sine PROM and a cosine PROM, respectively, containing a conversion of the signal to the sine and cosine function thereof. The digital sine and cosine components are in phase quadrature, and are applied to two digital-to-analog converters. The converters convert the digital data to analog signals which appear as bipolar currents at the two converter outputs. These current outputs are applied to two balanced modulators along with a divided RF carrier. The modulator outputs in phase quadrature (sine and cosine) are then combined in an adder to produce the desired frequency modulated radio frequency signal. The system uses digital techniques including PROMS containing phase history with a resolution of the input to the sine and cosine thereof. The output from the PROMS is converted to analog and used to drive the modulators with quadrature RF to give the affect of a rotating vector after combination thereof.
4. Objects of the Invention
A principal object of the present invention is to obtain FM phase modulated sweeps or "chirps" as they are also known, which may simulate target returns in a radar system, i.e. echoes of known transmitted pulses, or pulses of a known signature.
A further object of the present invention is to maintain coherence, and that is, the pulse compression signal must have phase coherence with the original signal generated as a narrow pulse.
Still another object of the present invention is to provide for generation of FM or pulse compression returns rapidly one after the other with practically no time in between.